Friday, August 27, 2010

More on the SCR system DEF Part two of two parts

As is usually the case in technical blogs, I'm not sure I made it clear that both Ford and Chevy are going to be urea consuming SCR systems on all their diesels.  So just sticking with one of them thru experience and brand loyalty will not get you the same truck or engine that you are familiar with.

Dodge on the other hand, is sticking with the exhaust gas recirculation technology of the past three years.  While you won't have to buy DEF and keep a fluid tank full in order to drive,  there are some major draw backs to staying in the EGR camp.

And it is worth noting that on their heavy trucks, they recognize that SCR is the way to go and add the system.  Are they just getting another few years out of the older technology in order to amortize their prior costs of research and development?

So I found specific arguments for each system out on the internet and will reprint them here.  From MHC Truck Source dot com.

"Selective Catalytic Reduction (SCR)  Urea injection

   It enhances thermal efficiency & fuel economy
   It reduces heat rejection and cooling system stresses, allowing for a
     smaller radiator and cooling fan and extending oil-drain intervals
   The smaller, lighter engine may equate to increased payload and less
     expensive (fuel) injection system.
   Can reduce emissions up to 90%
   Ideal where fuel economy and weight are primary considerations and
     trucks operate on main travel lanes.
   Availability of urea - searching for suppliers may add out-of-route miles
   Consumption of urea is unpredictable, since its mixing ratio varies with
     driving conditions.
   Vehicles will be fitted with a NOx sensor to ensure the urea level is not
     neglected. Failure to maintain the urea tank will result in a minimum 40%
     reduction in torque output if the additive runs out.
   The urea system cost doesn't really scale with engine size. It's a fixed cost
     and as you move down in engine size, it starts to account for a larger
     percentage of the engine cost.
   Least effective in stop start situations such as city operations where the
      constant acceleration creates the most NOx.
   Unknown price stability of urea.

Exhaust Gas Recirculation (EGR)

   Ideal for users running less traveled routes and those eager to avoid
     replenishing urea
   No additive, no extra tanks, and the loss of payload and fuel capacity
     associated with SCR
   No risk of experiencing a power down event due to NOx monitoring
   The addition of EGR coolers increases overall engine size and the
     additional heat loads could mean an increase of 10 to 30% cooling capacity
     might be required
   Technical risk related to the SCR catalyst and doser is eliminated.
   The simpler fueling requirements are easier for hired drivers, thus good for
     small-fleet driver retention.
   When combined with a DPF can reduce emissions up to 90%
   Engines are larger and possibly heavier, depending on power rating
   Larger radiator and fan are needed to handle small increase in heat
   The fuel cost is higher than the cost of fuel plus urea in an SCR system."

As I understand this, with SCR we will be able to make more simple, smaller engines without all the pollution stuff added on.  They will get the fuel economy that they did years ago and not cost as much to run.  But, you will have to buy urea solution and maintain the injection system.  Urea is corrosive and has to be handled in plastic tanks.  And I'm sure will do nasty things to your exhaust pipes.  Also, it is fertilizer used in agricultural applications and has experienced massive wild price swings over the years that I was an agricultural accountant.  It could very easily cost twice or three times as much next year.

But using the EGR system causes us to keep getting bigger engines that can handle being suffocated with their  own exhaust being re piped into the intake.  This robs power and makes for inefficient fuel consumption.  So the engine has to be bigger just to make the same power as the little one did years ago.  (This is why Dodge Cummins went from a 5.9L to a 6.7L in 2007.)

I have read several places that the fuel economy of the non suffocated SCR engine will more than pay for the DEF you have to purchase.  Even create an overall savings in the long run.

Ya, maybe you can save some of that money to put on new tail pipes at the end of a year or two.

This stuff fascinates me  because there is really no good answer.  Experience will bear out the actual cost over time.  And as Rick pointed out in his comment, I'm not sure I want to be a first year purchaser of this system.  Who's to say they won't come up with an entirely different system next year and you'll be stuck with a white elephant when no one except you needs DEF. 

So for me, I'm not too sure I will trade my pickup until this all settles down some.  The motorhome is EGR, which means I am purchasing way more fuel to keep the pollutants in check.  So perhaps I am spending just as much, if not more and just have no way to measure it.

Only time will tell....

So why am I using this a a blog topic?  It is because I spent today getting ready to go over to my doctor's office, and sit in the waiting room until my blood pressure went thru the sky, and then go get it tested!!  Oh, its really that high?  Imagine that.   But I have been having indigestion problems, so lets hope they can solve some of it for me.

Somehow that doesn't seem like much of a blog topic.  But studying new diesel trucks is always fun for me!

Retired Rod


  1. Your doing a greta job of researching this for all us Diesel jockeys, since I have less than 100.000 miles on my Duramax, it should be around a long time barring an accident. maybe be then like Rick said the problem will have enough time and use to be solved by whatever technolgy takes root. That urea price changing is a major concern. But we have invented substitutes for other products so may they will ahve an answer. Be safe out there Sam & Donna.

  2. Another interesting post on diesel technology, thanks Rod, it's good to know about these things a bit ahead of time.

    If this diesel stuff for our tanks is so great, maybe you should drink a gallon of it for your blood pressure? Just think of the mileage you'd get on walks!!

  3. I am probably the most technically challenged, mechanically challenged person in the blog world, and I find your posts on this subject to be fascinating. I can almost understand it, but I hope my old 2002 F350 will live as long as I do, so I won't be needing this new fangled stuff. Really, all I want is a little Jeep!


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